This invention relates generally to the field of printed document security, and more particularly to image data processing for the purpose of watermarking printed images.
It has been proposed to include watermark data in digital image data so that resulting printed images contain watermarks. The purpose of the watermarks may be to authenticate the printed images. That is, for example, fragile watermarks may be placed in original images to allow copies to be distinguished from originals; or robust watermarks may be placed in original images to allow the source of first or later generation copies to be ascertained. However, the process of printing an image and then scanning the printed image to retrieve the watermark results in scanned image data that is distorted relative to the original image data used in generating the printed image. This distortion in the “channel” by which the scanned image data is obtained from the original image data may compromise detection or other processing applied to the watermark in the scanned image data.